Cutaneous hyperaemia elicited by larvae of the cattle tick, Boophilus microplus

The responses of skin capillary blood flow (BF) to infestation by larvae of B. microplus have been determined using radioactive microspheres. Larvae were placed in gauze-covered rings glued to the closely clipped skin. In pilot experiments on three Brahman-cross calves which had prior exposure to ticks, BF measurements at intervals up to 25 h post-infestation revealed a continuing increase. 'High' rates of infestation (5000 larvae per ring) elicited up to an 80-fold increase in BF. In four Australian Illawarra Shorthorn (AIS) calves with prior exposure to ticks, BF had increased by 0.5 h post-infestation, it reached a peak at 24 h, and fell markedly by 48 and 75 h. In contrast, two AIS calves without prior tick exposure showed no significant increase in BF until 24 h and this increase continued until observations were terminated at 72 h. BF in the neck area (normally favoured by ticks) increased most in calves of highest resistance to ticks, but the flank area (normally unfavoured) showed no consistent difference in BF response between animals of different tick resistance. It is concluded that skin capillary hyperaemia is triggered by the host's immune response to larval attachment. Within the relatively narrow range of tick resistance in the present animals, the degree of hyperaemia was directly related to level of resistance to the tick in skin regions normally favoured by ticks and therefore it may play a role in determining resistance to ticks.